The wet line is the line of demarkation between the portion of the stock on the paper machine wire submerged in water and the portion having fibres extending above the depth of the water. Looking down at an acute angle to the wire on a paper machine as it is running one can see the glossy area where the water extends above the surface of the stock and a matte area at the opposite end of the wire where sufficient water has drained through the stock so that the fibres project above the water line and form what appears to be a matte finish. A very clear line of demarkation exists at the transition from the glossy area where the water flooded right through the stock to the matted area where the water level is below that of the fibres of the stock.
The wet line is generally kept within a certain area on the paper machine, its location being dependent upon the consistency of the stock, the position of the headbox slice, the pressure in the head box, the speed of the wire and the amount of suction applied through the wire to remove the water. In any event this line of demarkation or wet line is generally located well down the paper machine wire and usually is in the vicinity of the suction boxes.
An operator generally adjusts the various flows to the head box, the slice opening and the machine speed in order to position the wet line in a desired location. For example, if the wet line is too far down the machine there is a good chance there will be a break between the couch and the presses; whereas if the wet line is too close to the head box, the stock is obviously being drained too quickly.
Despite the fact that the wet line is useful to the operator for optimizing the setting of the paper machine, no means have been hitherto available to detect the location of this line and thus the operator must leave his control position to visually inspect the machine and then return and manually manipulate the controls to attempt to adjust the machine operation to position the wet line in the desired location. When steady state operation is attained some adjustment is occasionally necessary as a result of stock freeness changes, however, when grade changes are made i.e. basis weight changes or fibre changes (or furnish changes), the location of the wet line tends to shift drastically. Such movement of the wet line provides an indication that the change over to the other grade is not proceeding smoothly and that difficulties are being encountered and monitoring the changes in the location of the wet line can provide a guide to the operator indicating what may be wrong.